Research Summary Mutations in mitochondrial DNA (mtDNA) lead to a spectrum of neurodegenerative diseases for which no effective treatment exists. The most common of these is Leber hereditary optic neuropathy (LHON) caused by mutations in NADH dehydrogenase subunit genes (ND1, ND4 or ND6), which is complex I of the respiratory chain. Therapies for LHON in common with all disorders caused by mutated mtDNA are inadequate, in large part because of the barrier in delivering DNA into the organelle. We have successfully broken through this barrier by developing a pioneering adeno-associated virus (AAV) vector to which a mitochondrial targeting sequence (MTS) was appended to the viral capsid. The modified vector delivered the ND4 gene directly to the mitochondria for reversal of visual loss in mice with mutated G11778A ND4 responsible for more than half of all LHON cases, the rest caused by mutated ND1 and ND6. We will now design, modify and test the efficacy and safety of a clinically relevant vector for treatment of this mitochondrial disease by delivery of genes encoding the promoter and regulatory elements of a normal mitochondrially encoded human ND4 subunit to affected cells and tissues for rescue of cultured human LHON cells harboring the NADH dehydrogenase subunit 4 mutation causing LHON and then transgenic mice and primates with mutated ND4. The ND4 mouse we developed by injection of a MTS AAV containing mutated G11778A ND4 into the rodent blastocyst has visual loss progressing to blindness a year after birth, optic nerve head swelling followed by atrophy and degeneration of retinal ganglion cells, which are the characteristic hallmarks of LHON patients. Our goals are: (A) To facilitate translational studies for LHON by developing a clinical grade MTS AAV vector that accommodates the wild-type ND4 LHON gene and regulatory elements in a single AAV cassette. (B) To test expression to rescue respiration in cybrid cells and the visual system of mice and primates with mutated G11778A ND4. (C) To evaluate biological effects of intravitreal delivery of MTS AAV vectors in normal rodents and primates that result in mitochondrial gene transfer without adverse effects. (D) To develop an IND application for a single AAV containing normal ND4 for a future phase I/II clinical trial designed to restore the vision of patients with the commonest and most severe LHON mutation and prevent visual loss in their family members in a U10 application to follow successful completion of this R24.